Truly He is that light which illuminates the whole understanding of those who are capable of receiving truth, as is said in the thirty-sixth Psalm, “In Your light we shall see light.” For what other light of God can be named, “in which any one sees light,” save an influence of God, by which a man, being enlightened, either thoroughly sees the truth of all things, or comes to know God Himself, who is called the truth? Such is the meaning of the expression, “In Your light we shall see light;” i.e., in Your word and wisdom which is Your Son, in Himself we shall see You the Father. Because He is called light, shall He be supposed to have any resemblance to the light of the sun? Or how should there be the slightest ground for imagining, that from that corporeal light any one could derive the cause of knowledge, and come to the understanding of the truth? (From Book I)

The relationship of God with Christ can be helpfully compared to that of the sun to light, but the analogy has limitations.

The light of the physical sun illumines physical objects by sharpening contrast in a field of other objects.

The Light of Christ is the light of God. Light from light, it does not cause contrast or shadow, but brings forward the source of light itself.